No command specifically for that, but you can just run "exe ping <specific-ip>". If the IP/MAC isn't already in its ARP table (get sys arp), the FortiGate will naturally send out an ARP request to try and get it. (assuming the FortiGate has an IP in the same subnet, of course)
To have immediate feedback, you can run sniffer for ARP traffic on the relevant interface (diag sniffer packet <interface> "arp" 4 0 a).
[ corrections always welcome ]